The present invention relates to a vehicle attitude control apparatus that can stabilize the movement of the vehicle.
When a vehicle is in an understeer or oversteer condition, the vehicle""s attitude is controlled through the control of the braking force or drive power applied to the vehicle. In other words, when the vehicle is in an understeer condition, the braking force applied to the wheels on the inside of the turn is made to exceed the braking force applied to the wheels on the outside of the turn, or the drive power applied to the outside wheels is made to exceed the drive power applied to the inside wheels. When a vehicle is in an oversteer condition, the braking force applied to the outside wheels is made to exceed the braking force applied to the inside wheels, or the drive power applied to the inside wheels is made to exceed the drive power applied to the outside wheels. In this way, a yaw moment that stabilizes the movement of the vehicle is generated.
In a vehicle in which the steering angle is changed in accordance with the movement of a steering actuator driven in response to the operation of a steering wheel, a target steering angle corresponding to the amount of operation of the steering wheel is sought, and the steering angle is subjected to feed forward control so that the actual steering angle will match the target steering angle.
Conventionally, this braking force control and steering angle control have been performed independently of each other. As a result, where the movement of the vehicle changes due to the application of braking force control, it has been necessary to subject the steering angle to feedback control so that behavior index value corresponding to the change in the movement of the vehicle, such as the yaw rate, matches the target behavior index value corresponding to the amount of operation of the steering wheel, such as the target yaw rate.
However, the problem arises that the stability of the vehicle""s behavior is reduced due to the time lag between the change in the vehicle""s behavior due to the application of braking control and the feeding back of the behavior index value corresponding to the change in the vehicle""s behavior to the steering control.
Furthermore, the problem also arises that the yaw moment that stabilizes the movement of the vehicle declines, when the linear region in which the wheel lateral slip angle is proportional to the cornering force can no longer be maintained due to an excessive increase in the steering angle in order to eliminate the understeer condition, or when the braking force applied to the inner wheels becomes excessive.
Conventionally, when a vehicle is in an understeer condition, only the braking force applied to the inside rear wheel is increased, and the braking force applied to the inside front wheel is not increased. Furthermore, when a vehicle is in an oversteer condition, only the braking force applied to the outside front wheel is increased, and the braking force applied to the outside rear wheel is not increased. This is based on the fact that an increase in the braking force on the inside front wheel in an understeer situation or on the outside rear wheel in an oversteer situation is thought to have a negative impact on the stability of the vehicle""s behavior. In other words, if the braking force on the inside front wheel is increased when the vehicle is in an understeer condition or the braking force on the outside rear wheel is increased when the vehicle is in an oversteer condition, the yaw moment that stabilizes the vehicle""s behavior is thought to decline. However, conventional braking force control cannot sufficiently stabilize the vehicle""s behavior, and further vehicle movement stabilization is desired.
An object of the present invention is to provide a vehicle attitude control apparatus that resolves the above problems.
A first aspect of the present invention comprises a vehicle attitude control apparatus that can individually control the braking force applied to the right and left wheels so that when the vehicle is in an understeer condition, the braking force applied to the inside wheels is larger than the braking force applied to the outside wheels, and when the vehicle is in an oversteer condition, the braking force applied to the outside wheels is larger than the braking force applied to the inside wheels, such apparatus includes an operation member, a steering actuator that is driven in accordance with the operation of the operation member, means for transmitting the movement of the steering actuator to the wheels so that the steering angle changes in response to this movement, means for seeking a behavior index value corresponding to the change in vehicle movement based on the change in the steering angle, means for seeking the amount of operation of the operation member, means for seeking a target behavior index value corresponding to the sought amount of operation of the operation member based on a stored relationship between the amount of operation and the target behavior index value, means for seeking the braking force applied to each wheel, means for seeking a braking force difference between the braking force applied to the inside wheels and the braking force applied to the outside wheels, means for calculating a steering angle set value corresponding to the sought target behavior index value and braking force difference based on a stored relationship between the target behavior index value, the braking force difference and the steering angle set value, means for calculating a steering angle correction value corresponding to the difference between the sought behavior index value and target behavior index value based on a stored relationship between this difference and the steering angle correction value, and means for controlling the steering actuator so that the steering angle matches the target steering angle which is the sum of the steering angle set value and the steering angle correction value.
A second aspect of the present invention comprises a vehicle attitude control apparatus that can individually control the drive power applied to the right and left wheels so that when the vehicle is in an understeer condition, the drive power applied to the inside wheels is larger than the drive power applied to the outside wheels, and when the vehicle is in an oversteer condition, the drive power applied to the outside wheels is larger than the drive power applied to the inside wheels, such apparatus includes an operation member, a steering actuator that is driven in accordance with the operation of the operation member, means for transmitting the movement of the steering actuator to the wheels so that the steering angle changes in response to this movement, means for seeking a behavior index value corresponding to the change in the movement of the vehicle that occurs based on the change in the steering angle, means for seeking the amount of operation of the operation member, means for seeking a target behavior index value corresponding to the sought amount of operation of the operation member based on a stored relationship between the amount of operation and the target behavior index value, means for seeking the drive power applied to each wheel, means for seeking a drive power difference between the drive power applied to the inside wheels and the drive power applied to the outside wheels, means for calculating a steering angle set value corresponding to the sought target behavior index value and drive power difference based on a stored relationship between the target behavior index value, the drive power difference and the steering angle set value, means for calculating a steering angle correction value corresponding to the difference between the sought behavior index value and the target behavior index value based on a stored relationship between this difference and the steering angle correction value, and means for controlling the steering actuator so that the steering angle matches a target steering angle which is the sum of the steering angle set value and the steering angle correction value.
In the first aspect of the present invention, when the vehicle is in the understeer condition, the vehicle yaw moment acting toward the inside of the turn is increased by increasing the braking force applied to the inside wheels such that it exceeds the braking force applied to the outside wheels, and when the vehicle is in the oversteer condition, the vehicle yaw moment acting toward the outside of the turn is increased by increasing the braking force applied to the outside wheels such that it exceeds the braking force applied to the inside wheels.
In the second aspect of the present invention, when the vehicle is in the understeer condition, the vehicle yaw moment acting toward the inside of the turn is increased by increasing the drive power applied to the outside wheels such that it exceeds the drive power applied to the inside wheels, and when the vehicle is in the oversteer condition, the vehicle yaw moment acting toward the outside of the turn is increased by increasing the drive power applied to the inside wheels such that it exceeds the drive power applied to the outside wheels.
According to the first aspect of the present invention, the steering actuator is controlled such that the steering angle matches the target steering angle, which is the sum of the steering angle set value and the steering angle correction value. The steering angle set value corresponds to the target behavior index value set in accordance with the amount of operation of the operation member, and the braking force difference between the right and left wheels. The steering angle correction value corresponds to the difference between the target behavior index value and the sought behavior index value. According to the second aspect of the present invention, the steering actuator is controlled such that the steering angle matches the target steering angle corresponding to the sum of the steering angle set value and the steering angle correction value. The steering angle set value corresponds to the target behavior index value reflecting the amount of operation of the operation member, and the drive power difference between the right and left wheels. The steering angle correction value corresponding to the difference between the target behavior index value and the sought behavior index value. Because the steering angle set value corresponds to the feed forward component of the target steering angle and the steering angle correction value corresponds to the feedback component of the target steering angle, feed forward control and feedback control are performed.
According to the first aspect of the present invention, because the steering angle set value is determined in accordance with both the amount of operation of the operation member and the braking force difference between the right and left wheels, when the vehicle movement is stabilized by the braking force control, the steering angle can be controlled by the feed forward control in accordance with the braking force difference. According to the second aspect of the present invention, because the steering angle set value is determined in accordance with both the amount of operation of the operation member and the drive power difference between the right and left wheels, when the vehicle movement is stabilized by the drive power control, the steering angle can be controlled by the feed forward control in accordance with the drive power difference. Therefore, according to both aspects of the present invention, the responsiveness of the control and the stabilization of vehicle movement can be attained more effectively than when the steering angle is controlled only by the feedback control in accordance with the behavior index value obtained based on the control of the braking force or the drive power.
According to the first and second aspects of the present invention, in a case where the vehicle""s behavior is stabilized by controlling the braking force or the drive power applied to the right and left wheels when the vehicle is in the understeer or oversteer condition, the vehicle movement can be stabilized by the feed forward control of the steering angle not only in accordance with the amount of operation of the operation member, but also in accordance with the difference in the braking force between the outside wheels and the inside wheels or the difference in the drive power between the outside wheels and the inside wheels.
The third aspect of the present invention is applied in a vehicle attitude control apparatus, in which the braking force applied to the right and left wheels can be individually controlled such that when the vehicle is in an understeer condition, the braking force applied to the inside wheels exceeds the braking force applied to the outside wheels, while when the vehicle is in an oversteer condition, the braking force applied to the outside wheels exceeds the braking force applied to the inside wheels.
The third aspect of the present invention includes an operation member, a steering actuator that is driven in accordance with the operation of the operation member, means for transmitting the movement of the steering actuator to the wheels so that the steering angle changes in response to this movement, means for seeking a behavior index value corresponding to the change in the movement of the vehicle that occurs based on the change in the steering angle, means for seeking the amount of operation of the operation member, means for seeking a target behavior index value corresponding to the sought amount of operation of the operation member based on a stored relationship between the amount of operation and the target behavior index value, means for controlling the steering actuator so that the behavior index value matches the sought target behavior index value, means for seeking a wheel lateral slip angle, means for seeking the braking force applied to the inside wheel that maximizes the vehicle yaw moment acting toward the inside of the turn, when the vehicle is determined to be in the understeer condition, based on a stored equation, and means for determining whether or not the size of the sought wheel lateral slip angle equals or exceeds a predetermined maximum value when it is determined that the vehicle is in the understeer condition, wherein when the sought wheel lateral slip angle equals or exceeds the predetermined maximum value, braking force control is performed so that the sought braking force is applied, and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is minimized, and when the sought wheel lateral slip angle is less than the predetermined maximum value, as the sought wheel lateral slip angle decreases, the amount of braking force control is reduced and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is increased.
Through this constitution, when the steering actuator is controlled so that the difference between the vehicle target behavior index value corresponding to the amount of operation of the operation member and the detected behavior index value is reduced, the occurrence of the understeer condition or the oversteer condition as a result of the change in the steering angle due to the movement of the steering actuator can be prevented, by the application of the braking force. In other words, the vehicle""s behavior can be stabilized by the integrated control of the steering angle and the braking force.
Furthermore, in a state where the size of the wheel lateral slip angle equals or exceeds the predetermined maximum value when the vehicle is in the understeer condition, the braking force is applied to the inside wheels in order to maximize the vehicle yaw moment acting toward the inside of the turn, and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is minimized. In a case where the size of the wheel lateral slip angle is less than the predetermined maximum value when the vehicle is in the understeer condition, as the size of the wheel lateral slip angle decreases, the amount of braking force is reduced and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is increased. In this way, when the vehicle is in the understeer condition, an excessive increase in the steering angle and the excessive application of braking force to the inside wheels can be prevented, so as to prevent a reduction in the yaw moment that acts to stabilize the vehicle. In addition, the amount of braking force stabilizing the vehicle behavior can be increased as the vehicle""s behavior becomes more unstable and the lateral slip angle becomes larger, without the need for complicated control process.
A fourth aspect of the present invention comprises a vehicle attitude control apparatus that can individually control the braking force applied to each of the four wheels such that the braking force applied to the inside wheels exceeds the braking force applied to the outside wheels when the vehicle is in an understeer condition, and the braking force applied to the outside wheels exceeds the braking force applied to the inside wheels when the vehicle is in an oversteer condition, such vehicle attitude control apparatus further characterised in that the control is performed such that braking force applied to the front and rear inside wheels is increased when the vehicle is in the understeer condition, and the braking force applied to the front and rear outside wheels is increased when the vehicle is in an oversteer condition.
The fourth aspect of the present invention is based on the discovery that the vehicle yaw moment acting toward the inside of the turn can be increased when the vehicle is in the understeer condition by increasing the braking force on the front inside wheel, and the vehicle yaw moment acting toward the outside of the turn can be increased when the vehicle is in the oversteer condition by increasing the braking force on the rear outside wheel. In this way, the yaw moment that serves to stabilize the vehicle can be increased beyond the level obtained in the conventional art.
It is preferred that the fourth aspect of the present invention includes an operation member, a steering actuator that is driven in accordance with the operation of the operation member, means for transmitting the movement of the steering actuator to the wheels so that the steering angle changes in response to the movement of the actuator, means for seeking a behavior index value corresponding to the change in the movement of the vehicle that occurs based on the change in the steering angle, means for seeking the amount of operation of the operation member, means for seeking a target behavior index value corresponding to the sought amount of operation of the operation member based on a stored relationship between the amount of operation and the target behavior index value, means for controlling the steering actuator so that the behavior index value matches the sought target behavior index value, means for determining whether the vehicle is in the understeer or the oversteer condition based on at least the sought behavior index value and the target behavior index value, means for seeking, based on a stored equation, the braking force applied to the front and rear inside wheels that will maximize the vehicle yaw moment acting toward the inside of the turn when the vehicle is determined to be in the understeer condition, and means for seeking the braking force applied to the front and rear outside wheels that maximize the vehicle yaw moment acting toward the outside of the turn when the vehicle is determined to be in the oversteer condition, based on a stored equation.
By this constitution, in a state where the steering actuator is controlled so that the difference between the vehicle target behavior index value corresponding to the amount of operation of the operation member and the detected behavior index value is reduced, the occurrence of the understeer condition or the oversteer condition as a result of the change in the steering angle due to the movement of the steering actuator can be prevented, by the application of the braking force. In other words, the vehicle""s behavior can be stabilized by the integrated control of the steering angle and the braking force.
In the fourth aspect of the present invention, it is preferred that the vehicle attitude control apparatus further includes means for seeking the vehicle body lateral slip angle in time series, wherein: when the sought behavior index value is less than the target behavior index value and the sought vehicle body lateral slip angle is changing so as to bring the behavior index value closer to the target behavior index value, it is determined that the vehicle is in the understeer condition; when the sought behavior index value is less than the target behavior index value and the sought vehicle body lateral slip angle is changing so as to move the behavior index value away from the target behavior index value, it is determined that the vehicle is in the oversteer condition; and when the sought behavior index value exceeds the target behavior index value, it is determined that the vehicle is in the oversteer condition.
In a state where the direction of the driver""s operation of the operation member is the same as the turning direction of the vehicle, when the sought behavior index value is less than the target behavior index value, the vehicle is in the understeer condition, while when the sought behavior index value exceeds the target behavior index value, the vehicle is in the oversteer condition.
However, in a state where the vehicle is in the oversteer condition, when the operation member is operated in the direction opposite the direction in which the vehicle is being turned in order to eliminate the oversteer condition, i.e., when so-called xe2x80x98counter-steeringxe2x80x99 is performed, a situation arises in which the sought behavior index value is less than the target behavior index value corresponding to the amount of operation of the operation member, even though the oversteer condition is not in fact eliminated. If the braking force is applied to eliminate the understeer condition in this situation, the steering angle control performed to eliminate the oversteer condition conflicts with the braking force control performed to eliminate the understeer condition.
According to the above constitution, in a state where the sought behavior index value is less than the target behavior index value, it is determined whether the vehicle body lateral slip angle is changing so as to bring the behavior index value closer to the target behavior index value or so as to move the behavior index value away from the target behavior index value. When the vehicle is in the oversteer condition, if the sought behavior index value is less than the behavior index value corresponding to the amount of operation of the operation member due to the counter-steering operation, the vehicle body lateral slip angle changes so as to move the behavior index value away from the target behavior index value, and therefore it can be determined that the vehicle is in the oversteer condition. In this way, in a state where the counter-steering is carried out to eliminate the oversteer condition, the braking force can also be applied to eliminate the oversteer condition. Therefore, the above conflict between the steering angle control and the braking force control can be eliminated and the vehicle""s behavior can be stabilized.
In the third aspect of the present invention, it is preferred that the vehicle attitude control apparatus further includes means for seeking a wheel lateral slip angle, and means for determining whether or not the size of the sought wheel lateral slip angle equals or exceeds a predetermined maximum value when the vehicle is determined to be in the understeer condition, wherein: when the sought wheel lateral slip angle equals or exceeds the predetermined maximum value, the braking force control is performed so that the sought braking force is applied, and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is minimized, and when sought wheel lateral slip angle is less than the predetermined maximum value, as the sought wheel lateral slip angle decreases, the amount of the braking force control is reduced and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is increased.
In this way, when the size of the wheel lateral slip angle equals or exceeds the predetermined maximum value in the understeer condition, the braking force that maximizes the vehicle yaw moment acting toward the inside of the turn acts on the inside wheels, and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is minimized. Furthermore, when the size of the wheel lateral slip angle is less than the predetermined maximum value in the understeer condition, as the size of the wheel lateral slip angle becomes smaller, the amount of control of the braking force is reduced and the amount of control of the steering actuator to ensure that the behavior index value matches the target behavior index value is increased. In this way, when the vehicle is in the understeer condition, an excessive increase in the steering angle and the excessive application of braking force to the inside wheels can be prevented, thereby preventing a reduction in the yaw moment that stabilizes the vehicle""s behavior. In addition, the amount of braking force for stabilizing vehicle""s behavior can be increased as the vehicle""s behavior becomes more unstable and the wheel lateral slip angle becomes larger, without the need for a complicated control process.
In the third and fourth aspects of the present invention, it is preferred that the vehicle attitude control apparatus includes means for determining whether at least one of the value corresponding to the size of the vehicle body lateral slip angle and the value corresponding to the size of the rate of change of the vehicle body lateral slip angle exceeds a predetermined positive number value, wherein: when neither the value corresponding to the size of the vehicle body lateral slip angle nor the value corresponding to the size of the rate of change of the vehicle body lateral slip angle exceeds the predetermined positive number value, the amount of control of the braking force and the amount of control of the steering actuator in connection with the wheel lateral slip angle are set at a fixed level regardless of the size of the sought wheel lateral slip angle.
In this way, because neither the steering angle nor the braking force fluctuates more than necessary to ensure that the behavior index value matches the target behavior index value, a deterioration in the feel of steering operation can be prevented.
In the third and fourth aspects of the present invention, it is preferred that the predetermined maximum value for the wheel lateral slip angle does not exceed the maximum value at which the linear region, in which the cornering force is proportional to the wheel lateral slip angle, is maintained.
In this way, when the vehicle is in the understeer condition, the linear region, in which the wheel lateral slip angle is proportional to the cornering force, can be maintained by prevention of an excessive increase in the steering angle, and thus unstable vehicle movement can be reliably prevented.
In the third and fourth aspects of the present invention, it is preferred that the stored equation is expressed as
FX=axc2x7xcexcxc2x7W/(r2+a2)xc2xd
in which FX represents the braking force, W represents the tire load on each wheel, xcexc represents the coefficient of friction between the road and the tire of each wheel, r is sought from the relationship r=FO/(xcexcxc2x7W) in which FO represents the non-braking cornering force, a is sought from the relationship d/2=axc2x7Lf, when the braking force on the front wheel is calculated in which Lf represents the distance between the front wheels and the vehicle""s center of gravity and d represents the front wheel tread, and a is sought from the relationship d/2=axc2x7Lr, when the braking force on the rear wheels is calculated in which Lr represents the distance between the rear wheels and the vehicle""s center of gravity and d represents the front wheel tread.
In this way, an appropriate braking force that stabilizes the vehicle""s behavior can be applied.
In the third and fourth aspects of the present invention, it is preferred that the vehicle attitude control apparatus includes means for seeking the braking force applied to each wheel, means for seeking the difference between the braking force applied to the inside wheels and the braking force applied to the outside wheels, means for calculating a steering angle set value corresponding to the sought target behavior index value and braking force difference based on a stored relationship between the target behavior index value, the braking force difference and the steering angle set value, and means for calculating a steering angle correction value corresponding to the difference between the target behavior index value and the sought behavior index value based on a stored relationship between this difference and the steering angle correction value, wherein the behavior index value is matched to the target behavior index value, by controlling the steering actuator so that the steering angle matches a target steering angle which is the sum of the steering angle set value and the steering angle correction value.
In this way, the steering actuator is controlled so that the steering angle matches the target steering angle which is the sum of the steering angle set value and the steering angle correction value. The steering angle set value corresponds to the target behavior index value corresponding to the amount of operation of the operation member and the braking force difference between the right and left wheels. The steering angle correction value corresponds to the difference between the target behavior index value and the sought behavior index value. The steering angle set value corresponds to the feed forward component of the target steering angle, and the steering angle correction value corresponds to the feedback component thereof, and therefore feed forward control and feedback control are carried out. In other words, because the steering angle set value is determined not only in accordance with the amount of operation of the operation member, but also in accordance with the braking force difference between the right and left wheels, when the vehicle""s behavior is stabilized by controlling the braking force, the steering angle can be controlled by the feed forward control in accordance with the braking force difference. Therefore, the responsiveness of the control can be improved and the stabilization of vehicle behavior can be performed more effectively than when the steering angle is controlled only by the feedback control in accordance with the behavior index value obtained based on the braking force control.
According to the third aspect of the present invention, the vehicle attitude control apparatus performs integrated control of the braking force and the steering angle without any conflict between them in the understeer and oversteer conditions, with preventing the steering angle and the braking force from becoming excessively large in the understeer condition, improving the responsiveness of the control, and stabilizing the vehicle""s behavior.
According to the fourth aspect of the present invention, the vehicle attitude control apparatus increases the yaw moment that stabilizes the vehicle""s behavior beyond the level obtainable in the conventional art, by controlling the braking force appropriately in the understeer and oversteer conditions, with improving the responsiveness of the control, and stabilizing the vehicle""s behavior by controlling the braking force and the steering angle integrally without any conflict between them.